Summary
Most older properties in Letchworth have solid brick walls. Houses with solid walls can have insulation boards applied to the walls either externally, internally, or both (e.g. internally on the front facade and externally on the back, or externally and internally on the same wall – see Hybrid wall insulation). Internall wall insulation (IWI) is then covered with plaster to protect and conceal the boards.
IWI can significantly improve the energy efficiency of a home but there are a number of considerations to be aware of, notably a loss of internal floor space and disruption for occupants. Condensation risks also increase with greater thicknesses of insulation so it is important to find the ‘sweet spot’ for insulation depth (see below for more information).
Benefits of IWI over external wall insulation (EWI) include:
- Preservation of the existing external facade which is important when it is necessary to preserve the heritage/character of the area (see further information below on Letchworth Design Principles).
- Flexibility of installation (can be installed room by room).
- Generally cheaper than EWI.
Cost/difficulty
HIGH COST – The cost will vary with the type of insulation, thickness, and complexity of building. Typically a two-storey mid-terrace dwelling will cost £6000 to £8000. According to data from BEIS, the estimated cost of internal wall insulation is between £55 and £140 per m2 – B&NES Council.
HIGH DIFFICULTY – IWI can be complex to install and it is recommended to seek professional advice from a retrofit coordinator/designer to create a whole house retrofit plan to assess the pros and cons and devise a plan to suit your needs.
Applicability in Letchworth
Applicable – No specific requirements.
Insights from ASBP
“If installing internal wall insulation on a solid wall, even a relatively thin layer of natural fibre insulation (40mm) can meet the Building Regulations threshold U-value of 0.70 W/(m2·K) and provide significant energy savings.
By looking beyond purely U-values and finding a ‘sweet spot’ for insulation thickness, more internal floor area can be retained and homeowners can get a faster payback on their initial investment.
What’s more, by utilising ‘breathable’ natural materials we can ‘lock up’ carbon, and reduce condensation risks, helping to secure the long-term health of the building fabric and improve occupant comfort.”
Read more here – https://asbp.org.uk/briefing-paper/finding-the-sweet-spot.
One more thing… If you are using breathable insulation materials, make sure to decorate with permeable plasters and paints to retain the vapour open qualities of the building fabric.”
Insights from People Powered Retrofit
“People Powered Retrofit conducted a survey and retrofit options study of a typical home in Letchworth. One of their modelling scenarios (see scenario 4 in this report) found that adding just 20-40mm of insulation internally can cut heat loss from the walls by more than half. The insulation also helps to raise the internal temperature of the walls which can improve comfort and reduce the risk of condensation and mould growth.”
Case study
Victorian House Retrofit Case Study
An in-depth case study of a deep retrofit project in Sheffield, using cork insulation board installed internally with an insulating lime hemp plaster.
Links to further information
- Insulation and retrofit – Finding the sweet spot (2024)
- Energy Efficiency Guide for Letchworth Garden City residents (2024), page 3
- HCCSP Retrofitting Your House Guide (2024), page 17
- Saltaire Retrofit Handbook (2024), page 36
- Retrofitting Your Home – Cambridge Guide (2022), page 16
- The Surrey Home Energy Improvement Guide (2025), page 51
Image credits – Back to Earth; Ikonografik Design Ltd.
This guidance is part of Walls
See more guidance notes on this topic below: